1. Field of Invention.
This invention relates to an exciting unit used in an internal magnet type meter, and more particularly, to such a unit wherein a cylindrical permanent magnet is mounted in a cylindrical yoke through a gap wherein the effective sides of the moving coil can turn.
One moving coil type meter is the internal magnet type meter. This type meter has an exciting unit wherein a permanent magnet is arranged in a cylindrical yoke forming a magnetic circuit. The internal magnet type meter can be made to have a smaller size and is less influenced by external magnetic fields than the external magnet type meter. The moving coil in the internal magnet type meter is so arranged that its effective sides can move in a gap formed of its permanent magnet and yoke, to enclose the magnet. A magnetic flux to be generated by the magnet interlinks with the effective sides of the moving coil in that gap.
If a current to be supplied to the moving coil is denoted as I and if the density of the magnetic flux caused to interlink with the effective sides of the moving coil in the gap is denoted as B, the angle.theta. of the coil of the moving coil type meter is expressed by the following equation (1), as is well known in the art, whether the magnet is of the internal type or external type: EQU .theta.=K.multidot.I.multidot.B (1)
wherein K is a constant determined by the number of turns, shape, etc, of the moving coil.
Thus, the angle of deflection .theta. of the moving coil type meter is proportional to the product of the current I supplied to the moving coil and the density of the magnetic flux B acting upon the effective sides of that coil.
A number of moving coil type meters are used for measuring quantities of electricity in various fields handling electricity and electrons. This makes it desirable to lower the cost and decrease the size in order to expand the commercial market therefor. In order to attain such goals, the number of parts used in such meters must be reduced and the number of machining steps must also be reduced.
If the magnetic flux density B in the gap is maintained constant within the range of the deflection angle .theta. of the moving coil, the deflection angle .theta. is proportional exclusively to the current I, as seen from equation (1), so that the scale distribution of the scale plate becomes even.
In order to clarify the improvement produced by the invention, the structure of the exciting unit of the internal magnet type meter currently in use will be first described.
2. Description of Prior Art
FIG. 1 depicts an exemplary internal magnet type meter used in the prior art and comprising a cylindrical yoke 1 , a holding frame 2 and a cylindrical permanent magnet 3 attached to holding frame 2. A moving coil 4 is arranged to enclose permanent magnet 3 and is sandwiched between upper and lower tension springs 5a, 5b through tension bands (not shown). Permanent magnet 3 carrying moving coil 4 is arranged in cylinder of yoke 1 to form between the outer circumference of yoke 1 a gap in which moving coil 4 is allowed to turn. This gap should have a length smaller in deviation than a predetermined value within the range of the angle of deflection .theta. of moving coil 4. For this, it is necessary that the cylindrical internal magnet 3 be positioned coaxially with cylindrical yoke 1. Holding frame 2 is formed with an arm 2a for attaching it to yoke 1. Arm 2a has its back formed with a stepped portion P by a turnery. This stepped portion P is fitted in the inner face Q of cylindrical yoke 1 so that yoke 1 and permanent magnet 3 may be concentric. The lower part of FIG. 1 shows the yoke 1 with inner face Q, which is shown disassembled from the upper part. When assembled, yoke 1 will fit about frame 2, magnet 3, coil 4, with stepped part P holding inner face Q, and the screws 6a,6b fitting in the screw holes.
On the other hand, internal magnet 3 is made immovable in the upward and downward directions and in the turning direction with respect to yoke 1 by means of the two screws 6a and 6b.
The internal magnet type meter thus constructed is featured by it small size and by its strength against external magnetic fields. However, this prior art meter has certain disadvantages, such as (1) in order that internal magnet 3 may be arranged concentrically with yoke 1, holding frame 2 is turned with reference to the outer circumference of magnet 2; and (2) in order that internal magnet 3 be held in fixed positions in the vertical and turning directions, two screws 6a,6b have to be used, and their thread holes must be formed in the yoke 1. Thus, the prior art exciting units must use a large number of parts and require for their construction a large number of machining steps. Accordingly, it is difficult to reduce cost and size of prior art meters.